1. Field of the Invention
The present invention relates to a light emitting device a light emitting element that emits fluorescent light or phosphorescent light upon application of electric field to a pair of electrodes of the element which sandwich a layer containing an organic compound (hereinafter, an organic compound layer), and to a method of manufacturing the light emitting device. Further, the present invention relates to a deposition apparatus for fowling an organic compound layer or the like.
2. Description of the Related Art
Light emitting elements, which use organic compounds as a light emitting member and are characterized by the thinness, lightweight, fast response, and direct current low voltage driving, are expected to be applied to next-generation flat panel displays. Among display devices, ones having light emitting elements arranged in matrix are considered to be particularly superior to conventional liquid crystal display devices for their wide viewing angle and excellent visibility.
It is said that light emitting elements emit light through the following mechanism: voltage is applied between a pair of electrodes that sandwich an organic compound layer, electrons injected from the cathode and holes injected from the anode are recombined at the luminescent center of the organic compound layer to form molecular excitons, and energy is released while the molecular excitons return to the base state to cause the light emitting element to emit light. Singlet excitation and triplet excitation are known as excitation states, and it is considered that luminescence can be conducted through either one of those excitation states.
Such light emitting devices having light emitting elements arranged in matrix can use passive matrix drive (simple matrix type), active matrix drive (active matrix type), or other driving methods. However, if the pixel density is increased, active matrix drive in which each pixel (or each dot) has a switch is considered advantageous because they can be driven at low voltage.
Organic compounds for forming a layer containing an organic compound (strictly, light emitting layer), which is the center of a light emitting element, are classified into low molecular weight materials and high molecular weight (polymer) materials. Both types of materials are being studied but high molecular weight materials are attracting more attention because they are easier to handle and have higher heat resistance than low molecular weight materials.
A conventional active matrix light emitting device includes a light emitting element in which an electrode electrically connected with a TFT over a substrate is formed as an anode, an organic compound layer is formed thereover, and a cathode is formed thereover. Light generated at the organic compound layer can be extracted at the TFT side through the anode that is a transparent electrode.
In view of the above, the applicants suggested an active matrix light emitting device including a light emitting element having a structure in which an electrode on a TFT side, which is electrically connected to a TFT over a substrate is formed as an anode, a layer containing an organic compound is formed over the anode, and a cathode that is a transparent electrode is formed over the layer containing an organic compound (a top emitting structure) (Reference 1: Japanese Patent Laid-Open No. 2004-6327, Reference 2: Japanese Patent Laid-Open No. 2004-63461, and Reference 3: Japanese Patent Laid-Open No. 2004-31201).
The present invention provides a structure and a manufacturing method of an active matrix light emitting device in which the active matrix light emitting device can be manufactured in a shorter time with high yield at low cost compared with conventional ones.
It is a feature of the present invention that a layered structure is employed for a metal electrode which is formed in contact with or is electrically connected to a semiconductor layer of each TFT arranged in a pixel area of an active matrix light emitting device. Further, the metal electrode is partially etched and used as a first electrode of a light emitting element. A buffer layer, a layer containing an organic compound, and a second electrode layer are stacked over the first electrode.
The metal electrode to be formed in contact with the semiconductor layer of the TFT is processed and used as a first electrode; thus, the steps of forming a first electrode can be omitted.
Further, in the invention, a first electrode obtained by partially etching a metal electrode may be one or two layers of a metal film in a region which is in contact with a buffer layer (namely, a light emitting region). In addition, three or four layers of the metal film may be formed in a region in which a contact hole which reaches a semiconductor layer of a TFT is provided. The first electrode of the invention is not limited to the structure in which the region having three or four layers of the metal film surrounds the light emitting region.
The first electrode of the invention has different number of layers depending on the parts, so that steps are formed at the boundaries between layers having different number of layers. The steps are covered with an insulator (referred to as a bank, a partition wall, mound, or the like). Incidentally, at least an upper end of the insulator is curved to have curvature radius; the curvature radius preferably 0.2 μm to 0.3 μm. The curvature radius is provided to obtain good step coverage; thus, a layer containing an organic compound or the like to be formed later can be formed with extremely thin thickness.
Further, by providing a buffer layer on the metal electrode, distance between the first electrode and a second electrode in the light emitting element can be increased; accordingly, a short circuit in the light emitting element due to irregularities on the surface of the metal electrode or the like can be prevented.
The buffer layer is a composite layer of an organic compound and an inorganic compound which can accept electrons from the organic compound. Specifically, the buffer layer is a composite layer containing a metal oxide and an organic compound.
Further, the buffer layer is preferable because of superior conductivity in addition to the effect which is considered to be obtained by adding an inorganic compound (greater heat resistance or the like).
Accordingly, the thickness of the buffer layer can be made thicker without increase in the drive voltage; thus, a short circuit in an element due to dust in forming the light emitting element or the like can be prevented, and the yield can be improved.
In a full color light emitting device having three kinds (R, G, and B) of light emitting elements, the light emission efficiency varies depending on the emission colors. Excess current has been necessarily supplied in a light emitting element having bad light emission efficiency in order to balance the luminance of the whole light emitting surface of the light emitting device, which has been imperfection causing acceleration of deterioration of the light emitting device.
In accordance with the present invention, by controlling the thickness of the buffer layer, the distance between the first electrode and each light emitting layer is controlled by controlling the layer provided therebetween thereby improving the light emission efficiency. An excellent image can be displayed with clear color light emitted from each light emitting element; thus, a light emitting device with low power consumption can be realized.
Such advantages obtained by providing a buffer layer can not be obtained using a conventional hole transporting layer in which an organic compound and an inorganic compound which do not electrically affect each other are simply mixed.
Further, the buffer layer has both characteristics of hole injecting (or hole transporting) characteristics and electron injection (electron transporting) characteristics. Accordingly, a buffer layer may also be provided between the layer containing an organic compound and the second electrode so that the first electrode, a first buffer layer, the layer containing an organic compound, a second buffer layer, the second electrode may be stacked in order.
A light emitting device according to the invention includes a light emitting element having a first electrode connected to a semiconductor layer of a thin film transistor over a substrate having an insulating surface; an insulator covering an end portion of the first electrode; a buffer layer over the first electrode; a layer containing an organic compound over the buffer layer; and a second electrode over the layer. The first electrode has a first region and a second region having different number of layers from the first region, a step is formed at a boundary between the first region and the second region, and the step is covered with the insulator.
A light emitting device according to the invention includes a light emitting element having a first electrode electrically connected to a semiconductor layer of a thin film transistor over a substrate having an insulating surface; an insulator covering an end portion of the first electrode; a buffer layer over the first electrode; a layer containing an organic compound over the buffer layer; and a second electrode over the layer. The first electrode has a first region and a second region having different number of layers from the first region, a step is formed at a boundary between the first region and the second region, and the step is covered with the insulator.
In any of the above structures of a light emitting device above, a light emitting device includes a pixel area provided with the plurality of light emitting elements and a driver circuit having a plurality of thin film transistors, and the driver circuit includes a wiring having a same stack as the second region.
Further in the structure of a light emitting device above, the buffer layer is provided in contact with the first region of the first electrode. The buffer layer contains a composite material of an organic compound and an inorganic compound, and the inorganic compound is one or more selected from the group consisting of titanium oxide, zirconium oxide, hafnium oxide, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide. The buffer layer contains a composite material containing an organic compound having hole transporting characteristics and an inorganic compound.
In the above structure, the first electrode includes a first region having a metal film indulging two layers and a second region having a metal film including four layers. Alternatively, the first electrode includes a first region having a metal film indulging two layers and a second region having a metal film including three layers. Still alternatively, the first electrode includes a first region having a single layer metal film and a second region having a metal film including two or more layers. The number of the steps can be reduced as the number of the layers in a stack is reduced, and the total manufacturing time can be reduced
In each of the above structure, the first electrode has a film mainly containing an element selected from the group consisting of Ti, TiN, TiSiXNY, Al, Ag, Ni, W, WSiX, WNX, WSiXNY, Ta, TaNX, TaSiXNY, NbN, MoN, Cr, Pt, Zn, Sn, In, and Mo; or an alloy or a compound mainly containing the above element; or a stack of the films.
For example, when the first electrode has a first region including a single layer of Ti and a second region having a metal film of a stack including two layers (a Ti layer and Al layer), the number of the steps for forming a film can be reduced. In the case where the first electrode is in contact with the drain region, the Ti film is preferable since it has low contact resistance with a semiconductor (silicon). Further, when an Al film is used for the metal film stacked in the second region, the first electrode can be a low resistance electrode.
Further, in the case of using a structure in which a first region of a W single layer and a second region of a metal film having a stack including two layers (a W layer and an Al layer), etching can be easily conducted since the W film and the Al film has different etching rate.
Further, in each of the above structures, the area of a light emitting area of a light emitting element is smaller than the area of the first region.
Further, in each of the above structure, the second electrode is a light-transmitting conductive film.
Further, a structure of the invention for realizing the above structure includes a method for manufacturing a light emitting device including a plurality of light emitting elements having a first electrode, a layer containing an organic compound over the first electrode, and a second electrode over the layer containing an organic compound, comprising the steps of: forming a semiconductor layer of a thin film transistor; forming an insulating film covering the semiconductor layer of the thin film transistor; forming an electrode formed with a stack of metal layers in contact with the semiconductor layer of the thin film transistor, over the insulating film; removing a part of the stack of the electrode to form a first region, a second region having more layers than the first region, and a step at a boundary between the first region and the second region; forming an insulator covering the step and the second region of the first electrode; forming a buffer layer in contact with the first region; forming the layer containing an organic compound over the buffer layer; and forming the second electrode which transmits light over the layer containing an organic compound.
The structure of the invention is not limited to a full color display device having a pixel area provided with three kinds of light emitting elements (R, G, and B). For example, a full color display device can be obtained by combining a light emitting element of white light emission and a color filter. Alternatively, a full color display device can be obtained by combining a light emitting element of single color light emission and a color conversion layer. A full color display device may be manufactured using a pixel area provided with light emitting elements of four or more colors, for example (R, G, B, and W).
The present invention further suggests a new deposition apparatus in which an evaporation source is moved while a substrate is moved. FIGS. 7A and 7B each show an example of a deposition apparatus of the invention.
FIG. 7A shows a deposition apparatus includes a film formation chamber provided with an deposition shield for keeping the sublimation direction of a deposition material and a plurality of openings. The deposition material is sublimated through the plurality of openings. An evaporation source which is movable in a direction perpendicular to the moving direction of a substrate (also referred to as a transfer direction) is provided under the deposition shield. Further, the width Wb of the deposition shield is longer than the substrate width Wa so that the thickness of a deposited film is uniformed.
A deposition apparatus according to the invention includes a means for moving a substrate in a first direction in a film formation chamber; a deposition shield which can control heating temperature, which is fixed to an internal wall of the film formation chamber; and an evaporation source under the deposition shield and a means for moving the evaporation source in a second direction perpendicular to the first direction under the deposition shield. The deposition shield has a rectangular shape having a wider width than the width Wa of the substrate, a plurality of openings are provided on a top surface of the deposition shield, and a deposition material evaporated from the evaporation source is deposited to the substrate through the plurality of openings provided on the deposition shield.
A setting chamber connected to the film formation chamber may be provided via a gate in order to supply the deposition material to a crucible of the evaporation source. FIG. 7A shows an example of providing two crucibles provided on the evaporation source. However, the number of crucibles is not limited in particular and three or more crucibles may be provided, or one crucible may be provided. Further, the plurality of crucibles provided on the evaporation source may be inclined so that the evaporation centers converges thereby conducting co-evaporation.
The present invention further relates to a method for manufacturing a light emitting device using the above deposition apparatus including a plurality of light emitting elements each provided with a first electrode, a layer containing an organic compound over the first electrode, and a second electrode over the layer containing an organic compound. The substrate is moved and the evaporation source is moved in a direction perpendicular to the moving direction of the substrate in a film formation chamber to form a layer containing an organic compound over the first substrate.
The deposition apparatus shown in FIG. 7A can be set as one part of a multi-chamber manufacturing apparatus. In the case where the deposition apparatus shown in FIG. 7A is connected to an in-line manufacturing apparatus, it is connected to a transfer chamber in which pressure can be reduced. In the case of using one deposition shield and one evaporation source for one film formation chamber, the substrate is preferably moved over the openings of the deposition shield plural times to obtain desired film thickness.
As shown in FIG. 7B, two deposition shields may be provided perpendicularly to the moving direction of the substrate, and an evaporation source is provided on each deposition shield thereby continuously deposit the same deposition material to form a film. Film formation speed can be improved with the use of such a deposition apparatus. Further, nonuniformity of the film thickness of the deposited deposition material can be reduced by moving the evaporation source. The two deposition shields are provided parallel to each other with enough distance therebetween. Further, as to the deposition apparatus shown in FIG. 7B, a desired film thickness can be obtained without reciprocating the substrate above the deposition shields. Accordingly, the substrate can be moved in one direction and the deposition apparatus is preferably applied to an in-line manufacturing apparatus in which a plurality of chambers are arranged and connected in series. The deposition apparatus shown in FIG. 7B can also transfer the substrate; in the case of connecting the deposition apparatus shown in FIG. 7B to an in-line manufacturing apparatus, the chamber is connected between two chambers in which pressure can be reduced.
Alternatively, different deposition materials may be set on two evaporation sources to continuously form stacked layers. For example, a first organic compound and an inorganic compound are separately set on two crucibles of a first evaporation source; a substrate is moved above the first evaporation source so that a buffer layer is deposited on the substrate. Subsequently, the substrate is moved over a second evaporation source in which a second organic compound is set on its crucible so that a light emitting layer can be deposited on the buffer layer.
Further, the present invention relates to another method for manufacturing a light emitting device using the above deposition apparatus, in which the light emitting device includes a plurality of light emitting elements each provided with a first electrode, a layer containing an organic compound on the first electrode, and a second electrode on the layer containing an organic compound over a substrate having an insulating surface. The method for manufacturing a light emitting device includes the steps of forming a semiconductor layer of a thin film transistor; forming an insulating film covering the semiconductor layer of the thin film transistor; forming an electrode formed with a stack of metal layers in contact with the semiconductor layer of the thin film transistor, over the insulating film; removing a part of the stack of the electrode to form a first region, a second region having more layers than the first region, and a step at a boundary between the first region and the second region; forming an insulator covering the step and the second region of the first electrode; forming a buffer layer in contact with the first region by moving the substrate in a film formation chamber while moving the first evaporation source in a direction perpendicular to the moving direction of the substrate; forming the layer containing an organic compound over the buffer layer by moving the substrate by moving the substrate in the film formation chamber while moving the second evaporation source in a direction perpendicular to the moving direction of the substrate; and forming the second electrode which transmits light over the layer containing an organic compound.
Through the above manufacturing steps, the number of the manufacturing steps can be reduced by continuously forming a buffer layer and a layer containing an organic compound in one film formation chamber.
Note that a light emitting device in this specification means an image display device, a light emitting device and a light source (including an illumination device). In addition, the light emitting device includes all of a module in which a light emitting device is connected to a connector such as an FPC (Flexible Printed Circuit), a TAB (Tape Automated Bonding) tape or a TCP (Tape Carrier Package), a module in which a printed wiring board is provided on the tip of a TAB tape or a TCP, and a module in which an IC (Integrated Circuit) is directly mounted on a light emitting element using COG technology.
An electroluminescent element includes an anode, a cathode, and a layer containing an organic compound creating luminescence (electroluminescence) by applying an electric field (hereinafter referred to as an EL layer). Luminescence in an organic compound includes luminescence that is obtained when a singlet-excited state returns to a ground state (fluorescence) and luminescence that is obtained when a triplet-excited state returns to a ground state (phosphorescence). A light emitting device manufactured using a manufacturing apparatus and a film formation method according to the invention can be applied to whichever of the cases using either luminescence.
A light emitting element including an EL layer (an electroluminescent element) has a structure in which the EL layer is interposed between a pair of electrodes. Typically, the EL layer has a layered structure in which a hole transport layer, a light emitting layer, and an electron transport layer are stacked in order. The structure provides extremely high light emission efficiency, and is employed for most of light emitting devices that are currently under research and development.
Further, the structure in which an anode, a hole injection layer, a hole transport layer, a light emitting layer, and an electron transport layer are stacked in order; or the structure in which an anode, a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, and an electron injection layer are stacked in order can be used. The light emitting layer may be doped with a fluorescent pigment. All these layers can be formed of low molecular weight materials only or of high molecular weight materials only. The term “EL layer” in this specification is a generic term used to refer to all layers interposed between the anode and the cathode.
In a light emitting device according to the present invention, the drive method for screen display is not particularly limited. For example, a dot-sequential driving method, a line sequential driving method, a plane-sequential driving method or the like can be employed. Typically, a line sequential driving method is employed and a time ratio grayscale driving method or an area ratio grayscale driving method is used suitably. A video signal inputted to a source line of the light emitting device may be an analog signal or a digital signal, and driver circuits and other circuits are designed in accordance with the type of the video signal as appropriate.
In accordance with the present invention, in the case of a full-color light emitting device having three or more kinds of light emitting elements, excellent images can be displayed with clear color light emitted from each light emitting element; thus, a light emitting device with low power consumption can be realized.